1. Field of the Invention
The teachings herein relate to seismic tools used in subterranean exploration, and in particular, to techniques for detection of seismic events.
2. Description of the Related Art
Subterranean formations may be monitored using one or more seismic receivers. The receivers may be geophones placed at the surface or submerged in wells or on the ocean floor. Also, the receivers may be hydrophones placed in those same locations, but sensitive to only certain types of waves. The receivers placed in wells may be shallow (usually above the formation of interest) or deep (at or below the formation of interest). Seismic receivers may be sensitive to seismic waves along a certain axis or those traveling on any axis. Likewise, the receivers may be sensitive to only certain types of seismic waves, or several types. Those sensitive to a certain axis of travel, called directional receivers, may be coupled with other directional receivers. For example, a directional receiver may be coupled with two other directional receivers in a set of three orthogonal receivers which collect information about the waves in three dimensions. This three-dimensional information may be rotated mathematically through the use of trigonometric functions in order to derive information as to wave travel in the x-axis, y-axis, and z-axis relative to gravity. Alternatively, mathematical rotation may provide translation of the data relative to a wellbore, a cardinal direction, or any other reference point.
Microseismic monitoring concerns passively monitoring a formation for seismic events which are very small. Such events may include the seismic effects generated in a formation by fracturing, depletion, flooding, treatment, fault movement, collapse, water breakthrough, compaction or other similar subterranean interventions or effects. One of the main problems with microseismic monitoring, as with other forms of seismic monitoring, is that of noise. With microseismic events, however, the problem is emphasized because the signal strength is generally very small. This means, in turn, that a small amount of noise which would not cause any significant effect as to a regular, active seismic survey causes a significant degradation of the signal to noise ratio in the microseismic survey.
The geology of the microseismic environment is also of interest. Different geological layers are composed of different materials which transmit seismic waves at different velocities. It will be appreciated that when a source occurs in a low-velocity layer, its transmission through to a higher-velocity layer will cause attenuation, as much of the wave energy is reflected back into the low-velocity layer. When a low-velocity layer is sandwiched between two high-velocity layers, the resulting reflections from above and below can make seismic waves within the low-velocity layer very difficult to interpret. Therefore, receivers may be placed in overlying layers in order to provide a more distinct, though attenuated, signal.
Microseismic surveys include receiving data from a receiver, detecting data which exceeds some threshold, and analyzing those over-threshold data in order to determine information about certain events. Data which does not meet the threshold is discarded or simply not recorded as noise data.
Microseismic data may be analyzed as a set, with several receivers providing data for a joint analysis. Data is collected from a receiver and related to the other data collected from other receivers in order to derive additional information about the formation. Information from three receivers, for example, may be triangulated in order to estimate the location of a seismic event.
What are needed are methods and systems for detection of seismic events, such as microseismic events, which permit saving of data for the seismic events.